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[LewEngBridewell]

When I open all my car windows to get a fly out it wont go.

But you still have a windscreen and a back window don't you? I'm wondering what would happen if those windows weren't there? The fly wouldn't stand a chance in a moving car surely? It'd be just the same as having a convertible, the only difference being there's a shell of some window-frames where there would've been glass.

[racechick]

When I open all my car windows to get a fly out it wont go.

But you still have a windscreen and a back window don't you? I'm wondering what would happen if those windows weren't there? The fly wouldn't stand a chance in a moving car surely? It'd be just the same as having a convertible, the only difference being there's a shell of some window-frames where there would've been glass.

I think the fly would lurk down round the footwells!

[LewEngBridewell]

When I open all my car windows to get a fly out it wont go.

But you still have a windscreen and a back window don't you? I'm wondering what would happen if those windows weren't there? The fly wouldn't stand a chance in a moving car surely? It'd be just the same as having a convertible, the only difference being there's a shell of some window-frames where there would've been glass.

I think the fly would lurk down round the footwells!

Yes, they are crafty like that.

[stonemonkey]

obviously it would get blown around like crazy. But the greatest factor of force from drag is velocity (squared) of the air in relation to the object, so high-speed streams of air suddenly filling the cabin will have a huge effect on the fly - much more-so than gradually accelerating from naught to whatever speed you're driving at. But even if a car were to hit an immovable structure like a brick wall so the car (and air within it) went from high-speed to naught almost instantly, I still think the fly would continue through the air toward the windshield. Of course, its speed would certainly be decreased by drag, but not enough to stop it with the car before it hit the windshield.

I'm willing to bet that if you had a fly mid-flight in a sealed jar or aquarium and began shaking it, it wouldn't hold its position very well. Rather, it would rattle around the sides of the jar. Or alternatively, if you tied a dead fly to the roof of a car by floss, you would see it lean toward the back of the car as you accelerate.

In the case of the car accelerating the acceleration of the fly will be slightly behind the acceleration of the car and it will move towards the rear slightly, the higher the acceleration, the more it will move towards the rear during the acceleration period, in the case of shaking a fly in a jar, the acceleration of the jar will probably be more than you'd get from a car and it's in a much smaller space so it doesn't have to travel as far before it hits the sides so yes you probably could rattle a fly around in a jar and you could be right about the car crashing and stopping instantly but the reason an airborne fly doesn't get pushed against the rear window when a car accelerates is because the friction of the air mass inside the car which is moving with the car will cause the fly to accelerate in the same direction as the car.

If you used your dead fly tied to a bit of floss as a pendulum how long do you think it would swing until air resistance stopped it compared to something like a ball bearing tied to a bit of floss?

[acosmichippo]

Obviously a ball bearing will swing much longer, but the dead fly would swing long enough for a live, unattached fly to float to the rear window.

Anyway, I think we're essentially on the same page, just I tend to think the fly will overcome a bit more air resistance than you do.

[smokin]

Force is mass x acceleration and if the car is moving at a constant speed there is no force acting on the occupants (fly included) to move them from their current positions. If the car accelerates or decelerates, a corresponding force acts upon the occupants but because the fly has far less mass than a passenger, the force acting on the fly is far less. This is where the frictional force caused by moving through air, the 'air resistance', comes into play. The friction is proportional to the surface area exposed and in the case of the fly, this force is much closer to the opposite force from the acceleration or deceleration than the air resistance acting on a passenger. If you brake hard everything in the car not tied down will move forwards but you'll move much faster and further than the fly. You'll be splattered against the windshield but the fly won't.

By the way, if everything was in a vacuum both you and the fly would be splattered against the windshield - but in a vacuum you'd be dead anyway.

Opening the windows only affects the air next to the open window. At some points, air will be forced into the car, blowing the fly away from the window. At others, air is drawn out of the car. It depends where the fly is in the car when you open the window as to whether it is blown further in or sucked out.

[stonemonkey]

Just came across this showing how light things behave in cars:

[youtube]XXpURFYgR2E[/youtube]

Unlike the balloons though, flies are heavier than air but still have a pretty low density and low mass to surface area so it doesn't take much for them to move with the air in the car..

[LewEngBridewell]

Wow this thread is a blast from the past

Interesting vid BTW

[What's Burning?]

Just came across this showing how light things behave in cars:

[youtube]XXpURFYgR2E[/youtube]

Unlike the balloons though, flies are heavier than air but still have a pretty low density and low mass to surface area so it doesn't take much for them to move with the air in the car..

I had to watch this video a few times to makes sure I understood my own explanation as to why it seems counter intuitive yet it makes perfect sense when you understand the principal.

I've had a rough couple of days.

[scotty]

Just came across this showing how light things behave in cars:

[youtube]XXpURFYgR2E[/youtube]

Unlike the balloons though, flies are heavier than air but still have a pretty low density and low mass to surface area so it doesn't take much for them to move with the air in the car..

I think that video is more a display of air pressure in action myself..

[What's Burning?]

Exactly, and the balloon effectively acts like the bubble on a level finding that lowest pressure area.

[stonemonkey]

Just came across this showing how light things behave in cars:

[youtube]XXpURFYgR2E[/youtube]

Unlike the balloons though, flies are heavier than air but still have a pretty low density and low mass to surface area so it doesn't take much for them to move with the air in the car..

I think that video is more a display of air pressure in action myself..

Yep, the pressure difference is caused by the inertia of the air mass.

Exactly, and the balloon effectively acts like the bubble on a level finding that lowest pressure area.

Not exactly, in the case of a bubble level it's the force of gravity pulling on the liquid more than the gas bubble, in this case it's the inertia of the air due to the g forces of acceleration acting more on on the air than the lighter balloon.

[What's Burning?]

Just came across this showing how light things behave in cars:

[youtube]XXpURFYgR2E[/youtube]

Unlike the balloons though, flies are heavier than air but still have a pretty low density and low mass to surface area so it doesn't take much for them to move with the air in the car..

I think that video is more a display of air pressure in action myself..

Yep, the pressure difference is caused by the inertia of the air mass.

interesting to think that what we're seeing is the difference in gas density. I'd like to have seen a balloon filled with air to for comparison... a nice control.

[stonemonkey]

Maybe think of it like a weighing balance tipped onto it's side and one side has a heavier weight.
When not moving nothing happens, when it accelerates the heavier side has more inertia so the balance will swing with the heavier side moving back and the lighter side moving forward (relative to the vehicle). The bigger the weight difference, the bigger the effect.

[stonemonkey]

Was at work today and thinking about this and came up with another way to think about it.

In a stationary car or one moving at a constant speed, up and down are relative to up and down with respect to the ground.
When the car accelerates or decelerates the g forces on everything within the car change what up and down are, the g force vectors and the gravity vector can be added. If you're standing on a train as it pulls away from the station you lean forward to a position where you're balanced and effectively your body is on the up/down line created by adding the g force vector (pushing you back relative to the train) to the gravity vector. The same with the helium balloon, it always wants to go up but the acceleration of the car changes what up is.

If there is constant acceleration then the balloon with pull forward and up, if you hold a ball bearing to the top of the string where it's tied to the balloon and let it go it will fall following the line of the string to the bottom.